1. Field of the Invention
The present invention generally relates to mobile communication systems.
Detailed descriptions of mobile communication systems can be found in the literature, in particular in Technical Specifications published by standardisation bodies such as in particular 3GPP (3rd Generation Partnership Project).
2. Description of the Prior Art
It is simply recalled that the general architecture of a mobile communication system essentially comprises, as illustrated in FIG. 1:                a Radio Access Network 1, or RAN,        a Core Network 4, or CN.        
The RAN comprises network elements such as base stations 2 and base station controllers 3. The RAN is in relation, on the one hand with mobile stations 5, via an interface 6, and on the other hand with the CN via an interface 7. The CN is in relation with external networks (not illustrated specifically). Within the RAN, the base stations communicate with the base station controllers via an interface 8.
In systems of UMTS type, the RAN is called UTRAN (UMTS Terrestrial Radio Access Network), the base stations are called Node B, the base station controllers are called RNC (Radio Network Controller), and the mobile stations are called UE (User Equipment). The interface 6 is called Uu interface, the interface 7 is called Iu interface, the interface 8 is called Iub interface, and an interface 9 between RNCs, called Iur interface, is also introduced. The interface 6 is also called radio interface, and the interfaces 7, 8, 9, are also called terrestrial interfaces.
Now, there is an evolution of RAN (for example UTRAN) architecture towards an architecture called E-RAN (for example E-UTRAN). This evolution is also called Long Term Evolution. For example, the E-UTRAN comprises network elements such as base stations. The base stations are called e-Node B. The interface between e-Node Bs is called X2 and the interface between one e-Node B and the CN (or Access Gateway) is called S1.
In the following, UTRAN will more particularly be considered, as an example of RAN to which the present invention applies; however it should be understood that the present invention is not limited to this particular example.
UTRAN performs a number of functions, including RRC (Radio Resource Control) functions, as specified in particular in 3GPP TS 25.331.
Broadcasting of system information from UTRAN to UEs in a cell is specified in 3GPP TS 25.331. The system information elements are broadcast in System Information Blocks SIBs (SIBs currently specified being called SIB 1, . . . SIB 18). A SIB groups together system information elements of the same nature. Different SIBs may have different characteristics, regarding their repetition rate and the requirements on UEs to re-read the SIBs. The system information is organised as a tree. A Master Information Block (MIB) gives references and scheduling information to a number of SIBs in a cell. The SIBs contain the actual system information.
A problem with such broadcasting of system information is the time it takes for a mobile station to acquire system information. This has a significant impact on the delays as perceived by the user, such as in particular the delays at call establishment, or cell reselection, or call re-establishment. Furthermore, the amount of system information is increasing, due in particular to technology evolutions, such as for example current deployment of new radio access technologies and/or current evolutions of Radio Access Network architecture, thereby still increasing such delays. This negatively affects user experience. Another problem is that such broadcasting of system information requires significant bandwidth, which does not correspond to efficient bandwidth use. Another problem is that such broadcasting of system information lacks flexibility, in particular the broadcast information cannot be optimised or adapted to each situation.